1. Field of the Invention
The present invention relates to a hot strip rolling mill system and rolling method by which small-scale production of hot strips can be realized with a compact structure of equipment.
2. Description of the Prior Art
As described in "Recent Hot Strip Manufacture Techniques in Japan", (published by Japan Steel Association, Aug. 10, 1987), p. 176 and pp. 6-10, for example, a typical hot steel plate rolling mill system (hereinafter referred to as "hot strip mill") is large-scaled such that a slab of 200 t is rolled by one or a plurality of rough rolling mills into a bar with a thickness of 20 to 40 mm, which bar is then rolled by tandem finish rolling mills comprised of 6 to 7 stands. Such a hot strip mill provides a yield of 3 to 4 million tons/year and is adapted for mass production. A 4-high mill of work roll driving type is employed as each of the rough rolling mills, and a 4-high or 6-high mill of work roll driving type is employed as each of the finish rolling mills.
Although a steel plate (hereinafter referred to as a "slab") fed to the rough rolling mill is generally of about 200 mm thick, there also is seen a slab of about 50 mm thick due to recent development of a thin slab continuous casting method. In the latter case, rough rolling mills become unnecessary and the hot strip mill is constituted solely by a group of finish rolling mills.
On the other hand, known as being of small-scale production type is the so-called Steckel mill comprising one reversible rough mill and a reversible mill provided with furnace coilers upstream and downstream thereof, as described, for example, in "Hitachi Review Vol 70, No. 6", (Jun. 25, 1988), pp. 67-72. The Steckel mill accompanies a disadvantage that holding the strip temperature and removing surface scale are difficult to achieve at the same time, but it has been widely used for rolling those strips such as stainless steel plates which are less likely to produce scale.
When the Steckel mill is applied to plain steel strip or the like, strip surface scale produced in the coiler furnaces must be removed by descaling jet water, which results in a problem of lowering the strip temperature.
Since product quality has to be sacrificed as mentioned above, applications of products are limited and examples of their use are small in all the wide world.
The current typical hot strip mill is of mass production type providing a yield of 3 to 6 million tons per year. Hitherto, there has naturally existed a demand for reducing the production scale and also reducing the equipment size correspondingly. Recent generation of iron scraps in a great deal of amount has put importance on recycling of those scraps, and such a concept that small-scale hot strip mills should be dispersedly installed for conveniently collecting the scraps rather than centralizing large-scale hot strip mills has prevailed in the world. Such a small-scale hot strip mill is simply called "mini hot". Thus, needs for optimum mini hots have become more stronger. Although the thin slab continuous casting method which has been focused on recently is intended for a mini hot by eliminating or lessening rough rolling, a group of finish rolling mills is still employed as it is conventionally.